Conformal coatings have been used for many years in the electronics industry to protect electrical assemblies from environmental exposure during operation. A conformal coating is a thin, flexible layer of protective lacquer that conforms to the contours of an electrical assembly, such as a printed circuit board, and its components. Conformal coatings protect circuits from corrosive chemicals (for example salt, solvents, petrol, oils, acids and environmental pollutants), humidity/condensation, vibration, current leakage, electromigration and dendritic growth.
Current conformal coatings are typically 25 to 200 μm thick. There are 5 main classes of conformal coatings, according to the IPC definitions: AR (acrylic), ER (epoxy), SR (silicones), UR (urethanes) and XY (paraxylylene). Of these 5 types, paraxylylene (or parylene) is generally accepted to offer the best chemical, electrical and physical protection. However, the deposition process is time consuming and expensive, and the starting material is expensive. Parylene is typically deposited using conventional chemical vapor deposition techniques widely known to those skilled in the art.
Parylene is polymer with the following structure:
Parylene is deposited using a three stage vapor deposition process. A solid precursor is heated under vacuum and sublimes. It is important to appreciate that parylene, although sometimes erroneously called “paraxylene,” is not in fact prepared from the compound paraxylene. In fact, the precursor is [2.2]paracyclophane:
The chemical vapor is then passed through a high temperature furnace at around 680° C., so that the precursor splits into a reactive monomer. This reactive monomer then feeds into a deposition chamber and polymerizes on the surface of the substrate. Typical coating thicknesses for parylene are between 5 and 25 microns.
The parylene deposition technique described above is not ideal because of the high cost of the starting material, the high thermal energy consumption during monomer generation, the high vacuum requirements and the low growth rate.
There is therefore a need for conformal coatings that offer similar levels of chemical, electrical and physical protection as parylene, but that can be manufactured more easily and cheaply. In addition, there are a number of other disadvantages that may be associated with the current conformal coatings. The techniques used to deposit the coatings require that the contacts by which the assembly is connected to other devices are masked prior to coating, to prevent the conformal coating from covering the contacts. Coated contacts would not be able to make an electrical connection to other devices, since the conformal coating can be thick and insulating.
Furthermore, it may be very difficult and expensive to remove the current conformal coatings if reworking of the electrical assembly is required. There is no possibility of soldering or welding through the coatings, without prior removal. Additionally, due to the liquid techniques that are generally used to deposit these conformal coatings, there is a tendency for defects, such as bubbles, to form in the coating. These defects reduce the protective capabilities of the conformal coating. A further problem with prior art conformal coatings is that, due to the liquid techniques used during coating, it may be difficult to deposit the coating underneath components on the assembly.